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Interpretation Letter 4-311(I) - Strobe Lights for Flashing Beacons

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U.S. Department of Transportation
Federal Highway Administration

400 Seventh St., S.W.
Washington, D.C. 20590

August 23, 2006

In Reply Refer To: HOTO-1

Mr. John I. Vaughn
Vice President
Signal Equipment Company
P.O. Box 78172
Shreveport, LA 71137-8172

Dear Mr. Vaughn:

Thank you for your July 28 letter requesting an official interpretation regarding the use of strobe type lights for flashing beacons. You asked whether flashing beacons, as described in Chapter 4K of the Manual on Uniform Traffic Control Devices (MUTCD), can use strobe type lights, such as those in the "light bars" of police vehicles, rather than standard incandescent or light emitting diode (LED) indications.

Section 4K.01 states that flashing beacons shall follow the provisions of Chapter 4D. Section 4D.18 governs the design, illumination, and color of signal sections, including those used for flashing beacons. Text in that section indicates that "references to signal lenses in this section shall not be used to limit signal optical units to incandescent lamps within optical assemblies that include lenses." It further states that "research has resulted in signal optical units that are not lenses, such as, but not limited to, light-emitting diode (LED) traffic signal modules. Some units are practical for all signal indications, and some are practical for specific types such as visibility-limited signal indications." However, Section 4D.18 does require that the Institute of Transportation Engineers (ITE) "Standards for Vehicle Traffic Control Signal Heads" shall be met in terms of the design, illumination, and color of signal indications, and that the intensity and distribution of light from each signal indication should also conform to that ITE standard. The ITE standards are based on circular incandescent and LED light sources and light bar type indications would not be in conformance with those standards.

Additionally, Section 4D.11 requires that "the light source of a flashing signal indication shall be flashed continuously at a rate of not less than 50 nor more than 60 times per minute. The illuminated period of each flash shall be not less than half and not more than two-thirds of the total flash cycle." Your proposed rapidly pulsed flash pattern does not meet this requirement.

Currently, the MUTCD does allow a "stutter-flash" pattern for in-roadway warning lights (see Chapter 4L). This is the only traffic control application for which this type of flash pattern is allowed. Other uses are considered experimental, and any jurisdiction wishing to use it must receive experimentation approval from the Federal Highway Administration (FHWA) as per Section 1A.10 of the MUTCD. At the present time, the Florida Department of Transportation and the city of St. Petersburg are jointly experimenting with rectangular-shaped flashing beacons that use strobing light bar type indications to evaluate effectiveness in comparison to standard flashing beacons. Initial results are promising, but the project will continue until after the rectangular beacons have been in place for at least one year to evaluate whether their effectiveness diminishes over time. Further, a formal research project to evaluate the use of a "stutter flash" pattern for flashing beacons comprised of LED signal sections will be conducted over the next year by the Traffic Control Devices Pooled Fund Study, a consortium of 17 States, the FHWA, and other organizations.

In summary, the type of lights and flashing pattern described in your letter does not comply with the current MUTCD and any consideration of revising the MUTCD to allow this type of light source and/or flashing pattern for flashing beacons will not occur until after research and experimentation efforts are completed and evaluated.

Thank you for writing on this subject.  If you have any questions, please feel free to contact Mr. Scott Wainwright of our staff at or call him at 202-366-0857. Please note that we have assigned your request the following official interpretation number and title: "4-311(I) - Strobe Lights for Flashing Beacons." Please refer to this number in any future correspondence on this matter.


/s/ Anthony T. Furst

Anthony T. Furst
Acting Director, Office of Transportation Operations